This application is based upon, claims the benefit of priority of, and incorporates by reference the contents of prior Japanese Patent Application No. 2001-256772 filed Aug. 27, 2001.
1. Technical Field of the Invention
The invention relates to a discharge lamp device for lighting a high voltage discharge lamp. Specifically, the device is applicable to an automotive headlight device employing a discharge lamp.
2. Description of Related Art
In general, among the discharge lamp devices is a vehicle-mounted discharge lamp device which comprises a DC/DC converter for boosting a voltage supplied from an external power source, an inverter circuit for converting the boosted voltage into an alternating current voltage, and a starting circuit for producing high voltage to begin lighting a discharge lamp.
This starting circuit is provided with a high voltage transformer for causing a spark discharge so that a breakdown occurs between the electrodes of the discharge lamp. The high voltage transformer is composed of a primary winding and a secondary winding, and the secondary winding is connected between the discharge lamp and the inverter circuit.
In addition, wiring extending from the high voltage transformer to the discharge lamp is covered with a shield sheath in order to prevent noise radiation resulting from restriking noises that occur when the current flowing through the discharge lamp alternates in direction. The shield sheath also prevents noise radiation resulting from the alternating current flowing through the wiring that leads to the discharge lamp, upon alternating-current driving of the discharge light by the inverter circuit. Additionally, for the prevention of noise radiation, the high voltage transformer and the electronic circuits connected to the high voltage transformer, such as the inverter circuit, are typically accommodated in an electronic circuit case made of metal and are grounded along with the shield sheath.
In the conventional configuration, the shield sheath structure causes ground stray capacitances not only of the wiring between the discharge lamp and the high voltage transformer but also of the high voltage transformer. Consequently, when the high voltage transformer produces a high voltage at the start of lighting, the voltage to be applied to the discharge lamp charges these ground stray capacitances while being boosted. Subsequently, when the voltage reaches a high voltage and is applied to the discharge lamp for breakdown, the electric charges of the ground stray capacitance, having been charged up, then flow as a surge pulse current. In some cases, semiconductor switching devices, and the like, in the inverter circuit for converting a direct current voltage into an alternating current voltage may be broken.
The present invention has been achieved in view of the foregoing, and it is thus an object thereof to provide a discharge lamp device which can reduce noise radiation and reduce the surge pulse current resulting from the shield sheath.
According to a first aspect of the present invention, a lighting control circuit is provided including: a DC/DC conversion circuit having a first transformer for boosting a direct current voltage from a direct current power source; an inverter circuit having a semiconductor switching device for converting the voltage boosted by the DC/DC conversion circuit into an alternating current voltage; a starting circuit having a second transformer for boosting to such a voltage so as to cause a breakdown between electrodes of a discharge lamp in starting up the discharge lamp; and an electronic circuit case for accommodating the DC/DC conversion circuit, the inverter circuit, and the starting circuit. A secondary winding of the second transformer of the starting circuit is connected between the discharge lamp and the inverter circuit connected to the discharge lamp. An electrode member is interposed between the second transformer and the electronic circuit case.
Consequently, the interposition of the electrode member between the second transformer and the electronic circuit case allows suppression of a stray capacitance lower than the ground stray capacitance in the conventional configuration where the second transformer and the electronic circuit case are grounded therebetween.
It is therefore possible to reduce the stray capacitance to be charged when the second transformer produces a high voltage during startup. Thus, after a breakdown occurs between the electrodes of the discharge lamp, the amount of discharge of the electric charges, having been accumulated in the stray capacitances up to then, can be reduced with a reduction in surge pulse current.
In another aspect of the present invention, the electrode member is connected to a low-voltage side of the secondary winding of the second transformer. Consequently, even if such a high voltage, so as to cause a breakdown between the electrodes of the discharge lamp, is produced by the second transformer during startup, the connection of the electrode member to the low-voltage side of the secondary winding of the second transformer can surely reduce the stray capacitance that occurs in the second transformer.
In another aspect of the present invention, the electrode member is interposed at least between the secondary winding of the second transformer and the electronic circuit case. That is, to reduce the stray capacitance that occurs in the second transformer, the electrode member only has to be interposed between the second winding, which produces a high voltage, and the electronic circuit case. This will decrease waste of the electrode member used to reduce the stray capacitance.
In another aspect of the present invention, the electrode member is formed by evaporating a metal layer onto an insulating film. Consequently, the electrode member to be interposed between the second transformer and the electronic circuit case can be fabricated at a low cost without increasing the complexity or number of parts of the discharge lamp device, in particular, around the electronic circuit case.
In another aspect of the present invention, the electrode member is folded in two to cover both sides of the second transformer accommodated in the electronic circuit case. Since the second transformer accommodated in the electronic circuit case is covered at both sides with the folded electrode member, the ground stray capacitance of the second transformer can be eliminated.
According to another aspect of the present invention, the lighting control circuit is connected directly to the discharge lamp. This eliminates the need for the wiring from the second transformer of the starting circuit, constituting the lighting control circuit, to the discharge lamp, i.e., the shield sheath. It is therefore possible to reduce the surge pulse current resulting from the shield sheath while simplifying the discharge lamp device.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.